This invention relates to therapeutic methods and compositions for the treatment of the cellular membrane magnesium binding defect, a defect associated with certain abnormal physiological states, e.g., sodium-sensitive essential hypertension and Type 2 insulin-resistant diabetes mellitus.
The applicant discovered, by studying essential, or primary, hypertension in humans and in two strains of rats with genetic hypertension, that a specific metabolic defect is critically involved with the occurrence of so-called “salt-sensitive”, i.e. sodium ion sensitive, hypertension. This defect is the decreased binding of the magnesium ion (i.e. Mg2+) within the plasma membranes of somatic cells, in particular smooth muscle cells.
As a direct consequence of this defect, the intracellular concentrations of the magnesium ion decrease while those of the sodium ion (i.e., Na+) tend to increase due ostensibly to the increased passive permeability of the cell membranes for the latter ion. If the mammal's ability to remove the excess Na+ from the intracellular compartment is also compromised, then, as a consequence, the intracellular concentration of calcium ion (i.e. Ca2') also increases and causes, in particular, the heightened contractility of the smooth muscle cells lining the peripheral blood vessels.
The contraction of these cells causes the lumens of these vessels to decrease and consequently their resistance to blood flow increases. To overcome this increased resistance, and thereby to maintain the requisite blood flow, the heart contracts more strongly, causing the pressure in the arteries to increase. This abnormal, increased blood pressure is recognized clinically as hypertension. Since this result stems directly from the seemingly increased passive permeability of the cell membrane to sodium ion, the hypertension is classified as being “sodium sensitive” and occurs in approximately 50 per cent of the essential hypertensive population which comprises about 25 per cent of the population of the United States.
One general treatment proposed for the control of the blood pressure in essential hypertensive patients is the restriction of their dietary intake of salt (i.e. NaCl) the major source of sodium ion for the body. This measure is somewhat beneficial if the hypertension is salt-sensitive. However, if the hypertension is “salt-insensitive”, the restriction of the salt content of the diet has no therapeutic value aside from the frequently observed, concomitant reduction of food intake, and may actually worsen the hypertension.
The applicant also demonstrated that the magnesium binding defect is caused by the lack, or at least the decreased concentration, of a component of normal blood plasma. When erythrocytes from either salt-sensitive, essential hypertensive humans or rats are incubated with blood plasma from analogous normotensive subjects, the magnesium binding defect in the plasma membranes of these cells is corrected and the abnormal concentrations of intracellular ions are normalized. The effective components of normal blood plasma are identified as the pentapeptide and its contained tetrapeptide which comprise the C-terminal region of the tachykinin known as “Substance P”, the first mammalian produced tachykinin to be isolated and identified. It is composed of eleven amino acids joined by peptide linkages in the following sequence: Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2 (SEQ ID NO:1). The derived pentapeptide and its contained tetrapeptide which correct the magnesium-binding defect have the following amino acid sequences, respectively: Phe-Phe-Gly-Leu-Met-NH2 (SEQ ID NO:2); and Phe-Gly-Leu-Met-NH2 (SEQ ID NO 3) The applicant obtained evidence to indicate that the “general amino acid sequence” at the C-terminal region of mammalian-produced tachykinins is: Phe-X(Phe, Val)-Gly-Leu-Met-NH2 (SEQ ID NO:4), which comprises those pentapeptides and tetrapeptides occurring in mammalian blood plasmas that are derived from the tachykinins. The applicant has observed that they prevent the occurrence of the magnesium-binding defect in plasma membranes of somatic cells and also has demonstrated their in vivo effectiveness in correcting this defect in rats and the associated, salt-sensitive, essential hypertension.
The occurrence of the magnesium-binding defect in erythrocyte membranes also antagonizes, or “resists”, the effect of insulin to promote the uptake of magnesium by these cells. The applicant has examined the erythrocytes from a number of patients with “adult onset” or Type 2 diabetes mellitus and has found the magnesium-binding defect to occur with a frequency greater than 90%. Thus, the magnesium-binding defect is a significant contributor to the causation of “insulin resistance”, which in patients with Type 2 diabetes mellitus is, in most cases, considered to be the initiating cause of their diabetes.
The combination of the relationships of the magnesium-binding defect to salt-sensitive, essential hypertension, and to the characteristic insulin resistance of Type 2 diabetes mellitus, suggests a possible critical relationship of this defect with the occurrence of pre-eclampsia and eclampsia since salt-sensitive hypertension, insulin resistance, and overt diabetes mellitus are among the prominent clinical features of these two life-threatening, physiological abnormalities of human pregnancy.
The pentapeptides and tetrapeptides discussed above occur in normal blood plasma and are believed to be derived in vivo by enzymatic degradation of the tachykinins produced by nerve tissue, as well as by other tissues. Their quantitation in blood plasma could provide useful information for the diagnosis of those pathological states with which the magnesium-binding defect is critically associated.
Ostensibly, these substances are also believed to be highly specific, naturally occurring, therapeutic agents in contrast to the relatively non-specific, therapeutic substances presently available for the treatment of abnormal physiological states such as salt-sensitive, essential hypertension. However, they are peptides and, as such, are generally observed to be metabolically unstable, and therefore are subject to the restricted routes of administration necessary for this class of substances. Consequently, this invention concerns the compositions and pharmacological applications of a new class of biologically stable, monopeptide compounds which are derived from butadienes, ethylenes, and propanes, which can be utilized to treat and/or to prevent those abnormal physiological states with which the magnesium-binding defect is critically associated.